How did the increased biomass in the sunlit dish arise (i. e., where did the additional mass come from and by what process)?

When plants are exposed to sunlight, they take the CO2 found around and process it in the process of photosynthesis and this creates other molecules, some organic and some not.  The additional biomass came from the C and O2 molecules which came as a result of the photosynthesis after the plant processed the CO2.